EN 1993 Braced Frame — Concentrically Braced Frame Design per Eurocode 3 & 8

Complete guide to concentrically braced frame (CBF) design per EN 1993-1-1 and EN 1998-1. Cross-bracing (X-bracing), chevron (V and inverted-V) bracing, and K-bracing configurations. Ductility classes DCM (medium) and DCH (high), behaviour factors q for CBF, capacity design requirements, brace slenderness limits, and gusset plate connection design. Worked 3-storey CBF example with HEA 200 braces in S355 steel.

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Braced Frame Configurations

K-bracing is not permitted in seismic design (EN 1998-1 Cl. 6.7.1) because brace buckling induces large moments at column mid-height, risking column failure.

Behaviour Factors q for CBF (EN 1998-1 Table 6.2)

For DCH, braces must have bar_lambda >= 1.3 to ensure ductile behaviour (yield in tension before buckling).

Capacity Design (EN 1998-1 Cl. 6.7.4)

Connections and adjacent members must resist the overstrength brace force:

N_ov,Rd = 1.1 x gamma_ov x N_pl,Rd,brace

Where gamma_ov = 1.25 (material overstrength), 1.1 = strain-hardening factor.

Worked Example — 3-Storey CBF with HEA 200 Braces

3 storeys at 4.0 m, 3 bays at 6.0 m. X-bracing both directions. S355, HEA 200. DCM, q = 4.0.

Overstrength connection design: N_ov,Rd = 1.1 x 1.25 x 2128 = 2926 kN

Drift failure means increased brace size or additional braced bay required.

Brace Section Selection Guide

Design Applications

Common Design Scenarios

This reference covers structural design scenarios commonly encountered in structural steel design practice:

• Strength verification: Check member or connection capacity against factored loads per the applicable design code
• Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
• Code compliance: Ensure design meets all provisions of the governing standard
• Connection detailing: Verify weld sizes, bolt quantities, and edge distances

Related Design Considerations

• System behavior: consider the interaction between members and connections
• Load paths: verify that forces can be transferred through the structure to the foundations
• Constructability: check that the design can be fabricated and erected practically
• Cost optimization: evaluate alternative sections or connection types for economy

Worked Example

Problem: Verify a typical steel member for the following conditions:

Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category

Design Check:

1. Determine governing load combination (LRFD or ASD per applicable code)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. Verify interaction if combined forces exist

Result: Use the results from the Steel Calculator tool to verify design adequacy.

Design Applications

Common Design Scenarios

This reference covers structural design scenarios commonly encountered in structural steel design practice:

• Strength verification: Check member or connection capacity against factored loads per the applicable design code
• Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
• Code compliance: Ensure design meets all provisions of the governing standard
• Connection detailing: Verify weld sizes, bolt quantities, and edge distances

Related Design Considerations

• System behavior: consider the interaction between members and connections
• Load paths: verify that forces can be transferred through the structure to the foundations
• Constructability: check that the design can be fabricated and erected practically
• Cost optimization: evaluate alternative sections or connection types for economy

Worked Example

Problem: Verify a typical steel member for the following conditions:

Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category

Design Check:

1. Determine governing load combination (LRFD or ASD per applicable code)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. Verify interaction if combined forces exist

Result: Use the results from the Steel Calculator tool to verify design adequacy.

Frequently Asked Questions

What Australian Standard governs structural steel design?

AS 4100-2020 (Steel Structures) is the primary standard for structural steel design in Australia. It covers all aspects of design including member capacity, connections, serviceability, and fire resistance. The standard uses a limit states design philosophy with resistance factors (φ) applied to nominal capacities. Companion standards include AS/NZS 3679.1 for hot-rolled sections, AS/NZS 1554 for welding, and AS/NZS 4600 for cold-formed steel.

What are the common steel grades used in Australian construction?

The most common steel grades for Australian construction are Grade 300 and Grade 350 per AS/NZS 3679.1. Grade 300 (minimum yield 300 MPa for sections > 12 mm thick) is the standard for general structural applications. Grade 350 (minimum yield 340 MPa for sections > 12 mm) is used where higher strength reduces weight. Grade 400 and Grade 450 are available for specialized applications requiring higher strength-to-weight ratios.

How does AS 4100 compare to AISC 360?

Both AS 4100 and AISC 360 use limit states design (LRFD) principles. Key differences include: AS 4100 uses a single "capacity factor" φ approach rather than separate φ for different failure modes; AS 4100 specifies distinct buckling curves for hot-rolled and welded sections; the moment capacity formula in AS 4100 uses αm factor directly rather than Cb; and AS 4100 has more detailed provisions for slender sections and combined actions. Despite philosophical differences, both codes produce similar results for typical members.

Frequently Asked Questions

Key differences between DCM and DCH for CBF?

DCM: braces Class 1-3, bar_lambda <= 2.0, pinned beam-to-column joints acceptable. DCH: braces must be Class 1, bar_lambda 1.3-2.0, beam-to-column joints must be rigid or specifically detailed. Both use q = 4.0.

Why is K-bracing prohibited in seismic design?

K-bracing (per EN 1998-1 Cl. 6.7.1) induces unbalanced horizontal force at column mid-height when one brace buckles, causing large bending moments and potential column plastic hinging. Only X, V, and inverted-V bracing permitted.

Related Pages

• Moment Frame Design
• Framing Systems
• Seismic Design
• Brace Connection
• All European References

Educational reference only. CBF design per EN 1993-1-1:2005 and EN 1998-1:2004. Verify National Annex. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.

Design Resources

Calculator tools

• Bolted Connection Calculator
• Weld Capacity Calculator
• End Plate Moment Connection Calculator
• Fin Plate Shear Connection Calculator
• Gusset Plate Calculator

Design guides

• Bolted Connection Worked Example
• Bolted Connection Checklist
• Steel Connection Calculator Guide
• Weld Design Checklist
• EN 1993-1-8 Bolted Connection Worked Example

Reference pages

• EU Shear Tab
• UK Moment Connection